U.S. patent number 5,233,474 [Application Number 07/830,377] was granted by the patent office on 1993-08-03 for wide-angle lens system.
This patent grant is currently assigned to Asahi Kogaku Kogyo K.K.. Invention is credited to Jun Hirakawa.
United States Patent |
5,233,474 |
Hirakawa |
August 3, 1993 |
Wide-angle lens system
Abstract
A retro-focus wide-angle lens system having, in order from the
object side, a negative power front group and a positive power rear
group having a diaphragm stop. The front group comprises a positive
first lens element and a negative meniscus second lens element
having a convex surface directed towards the object. The rear group
comprises a positive third lens element, a negative fourth lens
element having an aspheric surface and a positive fifth lens
element. The negative power of the fourth lens element increases
toward the outer edge.
Inventors: |
Hirakawa; Jun (Tokyo,
JP) |
Assignee: |
Asahi Kogaku Kogyo K.K. (Tokyo,
JP)
|
Family
ID: |
14446725 |
Appl.
No.: |
07/830,377 |
Filed: |
January 31, 1992 |
Foreign Application Priority Data
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Feb 15, 1991 [JP] |
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3-106953 |
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Current U.S.
Class: |
359/717; 359/753;
359/793 |
Current CPC
Class: |
G02B
13/04 (20130101) |
Current International
Class: |
G02B
13/18 (20060101); G02B 13/04 (20060101); G02B
003/02 (); G02B 013/04 (); G02B 009/04 () |
Field of
Search: |
;359/717,793,753,714,766 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2114729 |
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Sep 1972 |
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DE |
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2727636 |
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Dec 1977 |
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DE |
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54-12728 |
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Jan 1979 |
|
JP |
|
0202414 |
|
Nov 1983 |
|
JP |
|
0178419 |
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Sep 1985 |
|
JP |
|
62-78520 |
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Apr 1987 |
|
JP |
|
0198006 |
|
Aug 1991 |
|
JP |
|
1088192 |
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Oct 1967 |
|
GB |
|
Primary Examiner: Arnold; Bruce Y.
Assistant Examiner: Lester; Evelyn A.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas
Claims
What is claimed:
1. A wide-angle lens system comprising, in order from an object
side to an image side:
a front lens component having a negative power and including, in
order from said object side to said image side, a positive first
lens element and a negative meniscus second lens element having a
convex surface directed toward said object side; and
a rear lens component having a positive power, said rear lens
component including a diaphragm stop, and further including, in
order from said object side to said image side, a positive third
lens element, a negative fourth lens element having an aspheric
surface and a positive fifth lens element, wherein the negative
power of said fourth lens element at each point along a radius of
said fourth lens element is a function of a distance between said
point and an optical axis, such that a negative power of said
fourth element increases in a radially outward direction.
2. A wide-angle lens system as claimed in claim 1, wherein a
distance between said aspheric surface and a spherical reference
plane at any given point along the aspheric surface is a function
of a distance between said point and the optical axis.
3. A wide-angle lens system as claimed in claim 2, wherein said
aspheric surface is shaped such that a distance between the
aspheric surface and the reference plane at an effective aperture
of the fourth lens element maintains a predetermined relation to a
distance between the aspheric surface and the reference plane at a
point along the aspheric surface half way between the effective
aperture and the optical axis.
4. A wide-angle lens system as claimed in claim 2, wherein said
lens system satisfies the following conditions:
where .DELTA.X1 is a distance between a point on the aspheric
surface at an effective aperture of the fourth lens element and
said spherical reference plane; .DELTA.X2 is a distance between a
point on the aspheric surface at one half of the effective aperture
and said spherical reference plane; fF is a focal length of the
front component; f is a focal length of the overall lens system;
and NRP is a refractive index at a d-line of a positive lens
element in the rear component.
5. A wide-angle lens system according to claim 1, wherein said
fourth lens element comprises a ground end polished glass substrate
having a spherical surface, and a synthetic resin layer overlying
said glass substrate and having anaspheric surface.
6. A wide-angle lens system according to claim 1 which satisfies
the following conditions:
where .nu.1 is an Abbe number of the first lens element; and N1 is
a refractive index of the first lens element at a d-line
thereof.
7. A wide-angle lens system as claimed in claim 2, wherein said
lens system satisfies the following condition:
where .DELTA.X1 is a distance between a point on the aspheric
surface at an effective aperture of the fourth lens element and
said spherical reference plane; and .DELTA.X 2 is a distance
between a point on the aspheric surface at one half of the
effective aperture and said spherical reference plane.
8. A wide-angle lens system as claimed in claim 2, wherein said
lens system satisfies the following condition:
where fF is a focal length of the front component; and f is a focal
length of the overall lens system.
9. A wide-angle lens system as claimed in claim 2, wherein said
lens system satisfies the following condition:
where NRP is a refractive index at a d-line of a position lens
element in the rear component.
10. A wide-angle lens system comprising, in order from an object
side to an image side:
a front lens component having a negative power and including, in
order from said object side to said image side, a positive first
lens element, and a negative meniscus second lens element having a
convex surface directed toward the object side; and
a rear lens component a positive power, said rear lens component
including a diaphram stop and further including, in order from said
side to said image side, a positive third lens element, a negative
fourth lens element having an aspheric surface and a positive fifth
lens element, wherein a distance between said aspheric surface of
said fourth lens element and a spherical reference plane at any
given point along the aspheric surface is a function of a distance
between said point and the optical axis.
11. A wide-angle lens system as claimed in claim 10, wherein said
aspheric surface is shaped such that a distance between the
aspheric surface and the reference plane at an effective aperture
of the fourth lens element maintains a predetermined relation to a
distance between the aspheric surface and the reference plane at a
point along the aspheric surface half way between the effective
aperture and the optical axis.
12. A wide-angle lens system as claimed in claim 10, wherein the
negative power at each point along a radius of said fourth lens
element is a function of a distance between said point and the
optical axis, such that negative power of said fourth element
increases in a radially outward direction.
13. A wide-angle lens system as claimed in claim 10, wherein said
lens system satisfies the following conditions:
where .DELTA.X1 is a distance between a point on the aspheric
surface at an effective aperture of the fourth lens element and
said spherical reference plane; .DELTA.X2 is a distance between a
point on the aspheric surface at one half of the effective aperture
and said spherical reference plane; fF is a focal length of the
front component; f is a focal length of the overall lens system;
and NRP is a refractive index at a d-line of a positive lens
element in the rear component.
14. A wide-angle lens system according to claim 10, wherein said
fourth lens element comprises a ground end polished glass substrate
having a spherical surface, and a synthetic resin layer overlying
said glass substrate and having an aspheric surface.
15. A wide-angle lens system according to claim 10 which satisfies
the following conditions:
where .nu.1 is an Abbe number of the first lens element; and N1 is
a refractive index of the first lens element at a d-line
thereof.
16. A wide-angle lens system as claimed in claim 10, wherein said
lens system satisfies the following conditions:
where .DELTA.X1 is a distance between a point on the aspheric
surface at an effective aperture of the fourth lens element and
said spherical reference plane, and .DELTA.X2 is a distance between
a point on the aspheric surface at one half of the effective
aperture and said spherical reference plane.
17. A wide-angle lens system as claimed in claim 10, wherein said
lens system satisfies the following condition:
where fF is a focal length of the front component, and f is a focal
length of the overall lens system.
18. A wide-angel lens system as claimed in claim 10, wherein said
lens system satisfies the following condition:
where NRP is a refractive index at a d-line of a positive lens
element in the rear component.
Description
BACKGROUND OF THE INVENTION
This application is based on and claims priority from Japanese
Patent Application No. Hei. 3-106953 filed Feb. 15, 1991, the
disclosure of which is incorporated by reference herein.
The present invention relates to a retro-focus type wide-angle lens
system suitable for use with cameras, such as single-lens reflex
cameras. More particularly, the present invention relates to a
wide-angle lens system having an aspheric surface.
In order to ensure adequate back focus, single lens reflex cameras
use retro-focus type wide-angle lens systems having a negative
power front lens group and a positive power rear lens group. (The
terms "group" and "component" are used interchangeably herein to
refer to at least one lens element.) For example, Unexamined
Published Japanese Patent Application No. 12728/1979 discloses a
compact Wide-angle lens system with a simple five-element
composition having an overall focal length F=2.8 and a half-view
angle of 37.degree.. In another example, Unexamined Published
Japanese Patent Application No. 78520/1987 discloses a
retro-focus-type wide-angle lens system having a five-element
composition, the front lens group of which includes an aspheric
surface.
However, the prior art wide-angle lens systems have several
problems. The system described in Unexamined Published Japanese
Patent Application No. 12728/1979 experiences large off-axis coma
and lateral chromatic aberrations, as a result of its simple
five-element composition. The system described in Unexamined
Published Japanese Patent Application No. 78520/1987 positions an
aspheric surface in the front lens group and away from the
diaphragm stop to correct for off-axis aberrations, field curvature
and distortion. However, this system cannot effectively correct for
astigmatism and lateral chromatic aberrations.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the problems
discussed above and to provide a five-element lens structure for a
retro-focus type high-performance wide-angle lens system by using
an appropriately shaped aspheric surface.
It is also an object to provide a wide-angle lens system
comprising, in order from the object side, a front group having a
negative power and a rear group having a diaphragm stop and a
positive power. The front group includes a positive first lens
element and a negative meniscus second lens element having a convex
surface directed towards the object. The rear group comprises a
positive third lens element and a negative fourth lens element
having an aspheric surface, wherein the fourth lens element's
negative power increases towards the edge. The rear group also
includes a positive fifth lens element. The lens system satisfies
the following conditions:
where .DELTA.X1 is the offset of the aspheric surface at the edge
of the effective aperture from the paraxial spherical surface along
the optical axis; .DELTA.X2 is the offset of the aspheric surface
at one half of the effective aperture from the paraxial spherical
surface along the optical axis; fF is the focal length of the front
group; f is the focal length of the overall lens systems; and NRP
is the refractive index at the d-line of a positive lens element in
the rear group.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more clearly understood from the following
description in conjunction with the accompanying drawings, in
which:
FIG. 1 is a simplified cross-sectional view of the wide-angle lens
system of Example 1;
FIG. 2 shows the aberration curves for the lens system of FIG.
1;
FIG. 3 is a simplified cross-sectional view of the wide-angle lens
system of Example 2;
FIG. 4 shows the aberration curves for the lens system of FIG.
3;
FIG. 5 is a simplified cross-sectional view of the wide-angle lens
system of Example 3;
FIG. 6 shows the aberration curves for the lens system of FIG.
5;
FIG. 7 is a simplified cross-sectional view of the wide-angle lens
system of Example 4;
FIG. 8 shows the aberration curves for the lens system of FIG.
7;
FIG. 9 is a simplified cross-sectional view of the wide-angle lens
system of Example 5;
FIG. 10 shows the aberration curves for the lens system of FIG. 9;
and
FIG. 11 illustrates the fourth lens element of FIG. 1 in detail
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a wide-angle lens system including a front lens
group 1 having a negative power and a rear lens group 2 having a
positive power. The rear lens group includes a diaphragm stop 3.
The front group comprises a positive first lens element 4 and a
negative meniscus second lens element 5 having a convex surface
directed towards the object. The rear group 2 comprises a positive
third lens element 6, a negative fourth lens element 7 having an
aspheric surface, and a positive fifth lens element 8. The negative
power of the fourth lens element 7 increases towards the outer edge
of the lens.
The rear group 2 possesses a strong positive power to focus the
light rays that have become divergent while passing through the
front group 1. However, using a rear group with strong positive
power increases the likelihood that spherical aberrations will not
be adequately corrected. To avoid this problem, element 7 (closest
to the diaphragm stop 3) in the rear group is constructed with an
aspheric lens surface S7. Thus, spherical aberrations can be
effectively corrected without substantially effecting the off-axis
light rays.
This aspheric surface S7 on the fourth lens element may be directly
worked from an optical glass material or indirectly shaped by
providing a thin synthetic resin layer on a spherical lens surface.
Examples 1 and 2 to be described hereinafter refer to the case
where an aspheric surface is directly worked from an optical glass
material. Examples 3, 4 and 5 refer to the case where an aspheric
surface is shaped by providing a thin synthetic resin layer
(surface S7 in the Tables) over a spherical surface (surface S8 in
the tables).
In the preferred embodiment of the present invention, the
wide-angle lens system must satisfy the following 3 conditions:
where .DELTA.X1 is the offset of the aspheric surface at the edge
of the effective aperture from the paraxial spherical surface along
the optical axis; .DELTA.X2 is the offset of the aspheric surface
at one half of the effective aperture from the paraxial spherical
surface along the optical axis; fF is the focal length of the front
group; f is the focal length of the overall lens systems; and NRP
is the refractive index at the d-line of a positive lens element in
the rear group.
A spherical aberration is a wave front aberration having a
geometric shape (i.e. the position at which a light ray intersects
the optical axis relative to the focal point) proportional to the
fourth power of the height (h) at which an incident ray contacts
the aspheric surface. Consequently, a spherical aberration can be
corrected by using an aspheric surface S7 (FIG. 1) having a shape
that is proportional to the fourth power of the height h. Thus, a
positive wave front aberration of a fourth-order shape may be
created by providing an aspheric surface of the fourth-order near
the diaphragm stop, wherein the negative power of lens element
having the aspheric surface increases towards the outer edge.
Condition (1) set forth above specifies the shape of the aspheric
surface S7 of the fourth lens element 7 in such a way that it is
generally proportional to the fourth power of the height of
incidence (h). If the fourth lens element has an aspheric surface,
the shape of which exceeds the upper limit of the equation in
condition (1), the overall system will overcorrect for marginal
on-axis light rays. If the shape of the aspheric surface on the
fourth lens element falls below the lower limit of condition (1),
the overall system will under correct for spherical
aberrations.
Condition (2) must be satisfied to ensure adequate back-focus and
compact size for the lens system. If the negative power of the
front group is made stronger than the lower limit of condition (2),
inward coma will develop at the second surface S2 of the second
lens element 5. Additionally, using a negative power above unit in
the front group will cause light rays to diverge in an increased
amount. This increased divergence will require a stronger positive
power in the rear group. However, increasing the positive power in
the rear group increases the likelihood of under-correction for
spherical aberrations.
Alternatively, if the negative power of the front group is made
weaker than the upper limit of condition (2), the overall size of
the lens system increases and it becomes more difficult to assure a
wide view angle.
Condition (3) specifies the refractive index of a positive lens
element in the rear group. The positive lens elements in the rear
group govern the overall power of the system and have a strong
positive power. Hence, by using an optical glass material of high
refractive index in these lens elements, the Petzval sum and,
hence, the field curvature can be reduced. If the refractive index
of the positive lens elements is too low to satisfy condition (3),
the Petzval sum becomes large, thereby increasing the likelihood
that field curvature will occur on the object side. Additionally, a
low refractive index requires a stronger curvature in the rear
group to ensure that the power of the rear group is sufficient.
However, as noted above, increasing the power of the rear group
increases spherical aberrations and coma effects.
In another embodiment, the wide-angle lens system is constructed to
satisfy the following conditions (4) and (5):
where .nu.1 is the Abbe number of the first lens element and N1 is
the refractive index of the first lens element at the d-line.
Conditions (4) and (5) specify the Abbe number and refractive index
of the first lens elements. Satisfying condition (4) contributes to
even more effective correction for lateral chromatic aberration,
and satisfying condition (5) contributes to even more effective
correction for distortion.
Five examples of the present invention are described below, in
which the shape of the aspheric surface S7 (also designated by an
asterisk) shall be represented by the following equation: ##EQU1##
where X is the coordinate in the direction of the optical axis; h
is the coordinate in the direction perpendicular to the optical
axis; C is the curvature (1/r); K is a conic constant; and An is an
aspheric coefficient (n=4, 6, 8, 10).
FIG. 11 illustrates the fourth element 7 (FIG. 1) in greater
detail. The paraxial spherical surface is designated at 20 and
corresponds to a spherical reference plane within the fourth
element. Spherical plane 20 intersects the optical axis at X=0,
such that the distance X between spherical plane 20 and the
aspheric surface S7 is designated by condition (1) set forth above.
The distance between plane 20 and surface S7 at any given point P
along the surface S7 is a function of the height h of the point
P.
EXAMPLE 1
FIG. 1 is a simplified cross-sectional view of a wide-angle lens
system according to Example 1 of the present invention. Specific
numerical data for this lens system are set forth in Table 1, and
the aberration curves for the system are shown in FIG. 2. Within
Table 1, r denotes the radius of curvature, d represents the
thickness of an individual lens or the air space between lens
surfaces, N is the refractive index, .nu. is the Abbe number, f is
the focal length, fB is the back focus, FNo. is the aperture ratio,
.omega. is the half view angle and An represents the aspheric
coefficients (n=4, 6, 8 and 10).
TABLE 1 ______________________________________ f = 100 fB = 127.2
FNo. = 1:2.8 .omega. = 37.6.degree. Surface No. r d N .upsilon.
______________________________________ S1 1351.00 10.43 1.58913
61.2 S2 -1509.94 0.35 S3 225.40 8.34 1.62041 60.3 S4 49.74 69.55 S5
84.27 38.23 1.77250 49.6 S6 -169.98 18.08 *S7 -84.60 6.95 1.78470
26.2 S8 124.89 2.29 S9 686.55 11.14 1.77250 49.6 S10 -64.47 K =
0.000 A4 = -7.926 .times. 10.sup.-7 A6 = 0.000 A8 = 0.000 A10 =
0.000 [log(.DELTA.X1/.DELTA.X2)/log2] = 4.0 fF = -115.8
______________________________________
EXAMPLE 2
FIG. 3 is a simplified cross-sectional view of a wide-angle lens
system according to Example 2 of the present INVENTION. Specific
numerical data from this lens systems are given in Table 2, and the
aberration curves for the system are shown in FIG. 4.
TABLE 2 ______________________________________ f = 100 fB = 128.1
FNo. = 1:2.8 .omega. = 37.6.degree. Surface No. r d N .upsilon.
______________________________________ S1 1293.91 10.43 1.69680
55.5 S2 -1752.26 0.35 S3 231.48 8.34 1.58913 61.2 S4 49.86 75.68 S5
82.91 35.13 1.74230 49.3 S6 -157.81 17.37 *S7 -85.22 6.95 1.76180
27.1 S8 121.14 3.17 S9 658.35 12.00 1.69680 55.5 S10 -61.86 K =
0.000 A4 = -9.030 .times. 10.sup.-7 A6 = 0.000 A8 = 0.000 A10 =
0.000 [log(.DELTA.X1/.DELTA.X2)/log2] = 4.0 fF = -123.6
______________________________________
EXAMPLE 3
FIG. 5 is a simplified cross-sectional view of a wide-angle lens
system according to Example 3 of the present invention. Specific
numerical data for this lens system are given in Table 3, and the
aberration curves for the system are shown in FIG. 6.
TABLE 3 ______________________________________ f = 100 fB = 126.8
FNo. = 1:2.8 .omega. = 37.6.degree. Surface No. r d N .upsilon.
______________________________________ S1 1208.36 10.42 1.60311
60.7 S2 -1208.36 0.35 S3 303.35 8.69 1.62041 60.3 S4 51.59 68.03 S5
78.76 38.61 1.77250 49.6 S6 -212.36 17.61 *S7 -90.34 0.35 1.54380
39.4 S8 -90.34 6.95 1.78472 25.7 S9 121.43 2.19 S10 663.25 11.23
1.77250 49.6 S11 -64.56 K = 0.000 A4 = -1.2972 .times. 10.sup.-6 A6
= 0.000 A8 = 0.000 A10 = 0.000 [log(.DELTA.X1/.DELTA.X2)/log2] =
4.0 fF = -114.3 ______________________________________
EXAMPLE 4
FIG. 7 is a simplified cross-sectional view of a wide-angle lens
system according to Example 4 of the present invention. Specific
numerical data for this lens systems are given in Table 4, and the
aberration curves for the system are shown in FIG. 8.
TABLE 4 ______________________________________ f = 100 fB = 127.2
FNo. = 1:2.8 .omega. = 37.6.degree. Surface No. r d N .upsilon.
______________________________________ Sl 2696.68 10.43 1.58913
61.2 S2 -1377.91 0.35 S3 232.48 8.34 1.62041 60.3 S4 56.59 80.65 S5
93.70 38.23 1.77250 49.6 S6 -194.71 23.16 *S7 -77.77 0.35 1.54380
39.4 S8 -77.77 6.95 1.80518 25.4 S9 162.23 3.67 S10 1095.14 13.48
1.77250 49.6 S11 -61.07 K = 4.035 A4 = 0.000 A6 = -1.735 .times.
10.sup.-10 A8 = 1.232 .times. 10.sup.-12 A10 + 0.000
[log(.DELTA.X1/.DELTA.X2)/log2] = 4.09 fF = -134.3
______________________________________
EXAMPLE 5
FIG. 9 is a simplified cross-sectional view of a wide-angle lens
system according to Example 5 of the present invention. Specific
numerical data for this lens system are given in Table 5, and the
aberration curves for the system are shown in FIG. 10.
TABLE 5 ______________________________________ f = 100 fB = 127.5
FNo. = 1:2.8 .omega. = 37.6.degree. Surface No. r d N .upsilon.
______________________________________ S1 1280.32 10.43 1.58913
61.2 S2 -1292.08 0.35 S3 251.03 8.34 1.62041 60.3 S4 50.21 69.57 S5
82.29 38.23 1.77250 49.6 S6 -179.98 17.55 *S7 -87.16 0.35 1.54380
39.4 S8 -87.16 6.95 1.78470 26.2 S9 119.65 2.09 S10 549.54 11.10
1.77250 49.6 S11 -65.54 K = 1.073 A4 = -9.526 .times. 10.sup.-7 A6
= 8.552 .times. 10.sup.-11 A8 = 0.000 A10 = 0.000
[log(.DELTA.X1/.DELTA.X2)/log2] = 3.98 fF = -114.6
______________________________________
As described above, an appropriate aspheric surface is provided in
a retrofocus-type wide-angle lens system having a simple
five-element composition, and the resulting wide-angle lens system
is compact and yet achieves high performance. without departing
from the spirit and scope of the invention as defined in the
appended claims.
* * * * *